Recently, the demand for video cassette tapes has been rapidly accelerating with the increasing popularity of video recording and playback machines as a contributory factor. Tape reels used in tapes of this type are made exclusively of plastic materials because of the ease with which such materials can be molded. Because of the purpose they serve, these reels have to be molded with extremely rigid tolerances in fabrication.
Plastic tape reels of quite satisfactory dimensional accuracy have been introduced to the market. However, since tape reels of this type are invariably required to pass the steps of thermal forming and curing, they tend to sustain sink marks during the step of curing. They have a common inherent problem that even the slightest deformation may possibly produce an adverse effect on the performance of the video recording and playback machine with which they are used.
As is widely known, the tape reel of this type comprises a pair of disc-shaped flanges connected axially to each other through the medium of an intervening hub. This hub consists of an outer cylindrical part adapted for a tape to be wound into a roll thereon and an inner cylindrical part disposed coaxially relative to the outer cylindrical part and provided therein with a space for admitting the insertion in the axial direction of a reel engaging shaft of the video recording and playback machine. One of the flanges is integrally molded at one end of the hub of such a construction, while the other flange is separately molded and afterward joined to the remaining end of the hub (which occurs as a free end at the time of molding) by means of ultrasonic welding or snapping engagement.
In the conventional tape reel, the portion which connects the inner cylindrical part and the outer cylindrical part of the hub in the radial direction has constituted an annular portion falling flush with the flange part of the integrally molded flange-and-hub combination. When this integral combination is molded in a molding die which has an injection gate at a position corresponding to the center of the free end of the hub part, it is imagined that the molten resin flows in diverse courses, some extending in axial directions from the outer edge of this annular portion to the edge of the outer cylindrical part and others extending in a branching manner in the radial directions to the periphery of the flange part. It is in the branched zone that the molded tape reel tends to sustain the aforementioned sink marks. Even when the edge portion supported by this annular portion is allowed to contract uniformly during the curing of the outer cylindrical part, the remaining axial end of the hub to which the separately molded flange is to be joined still tends to warp irregularly inwardly in the radial direction and impair the true circularity of the outer surface of the hub, possibly with the result that the produced tape reel will have to be rejected because the hub produces non-uniform rotation and causes an elongation in the tape would thereon.
In view of the shortcoming described above, there has been proposed a tape reel wherein the annular portion radially connecting the inner and outer parts of the hub is bent vertically throughout the entire circumferential extent so that the inner boundary of the annular portion joins one axial end of the inner cylindrical part and the outer boundary thereof joins the outer cylindrical part at a location halfway along the axial length thereof (Japanese Laid-open Patent Publication No. 97066/1980, U.S. Pat. No. 4,262,856).
When the paths for the flow of the molten resin to the outermost parts of the hub in the integrally molded hub-and-flange combination are considered with respect to the distances of travel from the aforementioned injection gate to the various points along the outermost boundary of the flange, they are found to vary because of the change in shape of the various parts of the hub and, consequently, the flow speed and flow volume of the molten resin proportionally vary from one path to another. Thus, it has been difficult to form the flange and the outer wall of the hub in exact circularity.